Optical bench on substrate

ABSTRACT

An optical bench on substrate includes a substrate and a trench formed inside the substrate and having a sloping side. A reflector layer is formed over the sloping side. An optical component is mounted over the substrate. The reflector layer is configured to reflect an electromagnetic wave to or from the optical component.

TECHNICAL FIELD

The present disclosure relates generally to an integrated circuit.

BACKGROUND

A multi-chip module package (MCM) integrates chips with different functions and made of different processes. Some MCMs utilize substrate materials based on ceramic or organic polymers, which, in certain configurations, may have insufficient coefficient of thermal expansion (CTE) matching to semiconductor chips and/or heat dissipation property. This causes potential reliability issues for III-V semiconductor material based optoelectronic chips and/or high power amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an optical bench on substrate according to some embodiments;

FIGS. 2A-2E are schematic diagrams of various steps of fabrication process of the optical bench on substrate in FIG. 1 according to some embodiments; and

FIGS. 3A-3C are schematic diagrams of various steps of another fabrication process of the optical bench on substrate in FIG. 1 according to some embodiments.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use, and do not limit the scope of the disclosure.

In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features.

FIG. 1 is a schematic diagram of an optical bench 100 on substrate according to some embodiments. The optical bench 100 includes a laser diode land a photo diode 2 mounted on a substrate 3. The laser diode 1 and the photo diode 2 comprise III-V semiconductor materials and operate on electromagnetic wavelengths in the range of 450 nm 1700 nm in some embodiments. The substrate 3 can comprise any suitable material, such as silicon. An etching hard mask 4 comprises SiN or SiO₂ and can achieve at least 30 nm etch depth in some embodiments. The etching hard mask layer 4 over the area for a trench 21 and/or an optical waveguide 19 is removed. In some examples, the etching hard mask layer 4 comprises SiN of at least 30 nm in thickness. In further examples, the etching hard mask layer 4 comprises SiO₂ of at least 100 nm in thickness. A reflector layer 5 comprises at least one of Cu, Al, Ag, or Au, multi-layered dielectrics, or any other suitable material having a reflective property at desired electromagnetic wavelengths. In some embodiments, the reflector layer 5 has at least 90% reflectivity at selected wavelengths.

A dielectric layer 6 can comprise SiO₂ or other low-k dielectric materials such as porous SiO₂, organic polymers such as polymide or Polybenzobisoxazole (PBO), or hybrid-organic polymers such as polysiloxane. To achieve high performance at radio frequency (RF) and microwave frequency, the thickness of the dielectric layer 6 is at least 300 nm for high resistance silicon substrate (resistivity >3000 ohm-cm) in some embodiments. The thickness of the dielectric layer 6 is at least 1 μm for normal wafers (resistivity is from 1 ohm-cm to 10 ohm-cm) in some embodiments.

A redistribution layer (RDL) 7 formed over the substrate 3 is a metal layer on a chip that allows the Input Output (IO) pads of an integrated circuit available in other locations. The RDL 7 comprises Al, Cu, or any other suitable electrically conductive material, and has more than 1 μm thickness for high speed applications over 2 Gbps in some embodiments. A passivation layer 8 comprises SiO₂, SiON, SiN, multi-stacks of these materials, or any other suitable materials. The thickness of the passivation layer 8 is about 200 nm-800 nm for pad protection in some embodiments.

A bottom cladding layer 9 comprises SiO₂/SiON formed by plasma-enhanced chemical vapor deposition (PECVD) in some embodiments. In other embodiments, spin-on dielectrics or polymers can be used to form the bottom cladding layer 9. The thickness of the bottom cladding layer 9 is at least 500 nm in some embodiments to prevent optical leak. A core layer 10 comprises SiON/SiN formed by plasma-enhanced chemical vapor deposition (PECVD) in some embodiments. In other embodiments, spin-on dielectrics or polymers can be used to form the core layer 10. The thickness of the core layer 10 is at least 15 m in some embodiments. A top cladding layer 11 comprises SiO₂/SiON formed by plasma-enhanced chemical vapor deposition (PECVD) in some embodiments. In other embodiments, spin-on dielectrics or polymers can be used to form the top cladding layer 11. The thickness of the top cladding layer 11 is at least 500 nm in some embodiments to prevent optical leak. In some embodiment, an optical fiber can be placed in the trench 21 as the waveguide 19.

The bottom cladding layer 9, the core layer 10, and the top cladding layers 11 form the waveguide 19 inside a trench 21 as an optical link medium for the electromagnetic wavelengths used by the laser diode 1 and/or the photo diode 2. The refractive index of the core layer 10 is higher than that of the bottom and top cladding layers 9 and 11, and the refractive index difference is at least 0.02 in some embodiments to prevent optical leak. In one example, three polymer layers for the bottom cladding layer 9, the core layer 10, and the top cladding layer 11 are deposited by a spin on process, then a lithography process is used to define the optical waveguide 19. An optical path 20 shows an exemplary light path of light (electromagnetic wave) emitted from the laser diode 1, reflected by a sloping side of the reflector layer 5, through the optical waveguide 19, reflected by another sloping side of the reflector layer 5, then to the photo diode 2.

An under bump metallization (UBM) layer 12 comprises any suitable under-bump metallurgy, e.g., Cu/Ni. A solder bump layer 13 can comprise lead-free solder or gold bumps in some embodiments. The solder bump layer 13 comprises micro bumps for flip-chip bonding with semiconductor-based optical and electrical chip in some embodiments. The overall thickness for the UBM layer 12 and the solder bump layer 13 is 1 μm-15 μm in some embodiments. Through substrate vias (TSVs) 14 formed through the substrate 3 comprises Cu or any other suitable electrically conductive materials. The TSVs 14 can be used to provide backside electrical connections, and can be fabricated using any suitable methods and materials known in the art.

Another dielectric layer 15 can comprise SiO₂ or other low-k dielectric material such as porous SiO2, organic polymers such as polymide or Polybenzobisoxazole (PBO), or hybrid-organic polymers such as polysiloxane. To achieve high performance at radio frequency (RF) and microwave frequency, a thickness of the dielectric layer 6 is at least 300 nm for high resistance silicon substrate (resistivity >3000 ohm-cm) in some embodiments. The thickness is at least 1 μm for normal wafers (resistivity is from 1 ohm-cm to 10 ohm-cm) in some embodiments. A backside redistribution layer (RDL) 16 comprises Al, Cu, or any other suitable electrically conductive material, and has more than 1 μm thickness for high speed applications over 2 Gbps in some embodiments.

The trench 21 has sloping sides with a slope angle θ ranging from 42° to 48° and has a depth more than 30 μm in some embodiments to accommodate the diverged optical beam from the laser diode 1, e.g., a vertical cavity surface emitting laser (VCSEL). Some laser diode 1 has a beam diversion angle of about 20°-30° with a beam size of about 10 μm-15 μm.

The integrated optical bench 100 on substrate allows coupling the light from the laser diode 1 to the reflector layer 5 and into the waveguide 19. It also leads the light out of waveguide 19 to the reflector layer 5 to be received by the detector diode 2. The optical bench 100 on substrate can also be implemented with one portion on either side of the line 22. For example, in one or more embodiments, the optical bench 100 includes the transmitting portion on the left side of the line 22 and having the laser diode 1 as a transmitter. In one or more embodiments, the optical bench 100 includes the receiving portion on the right side of the line 22 and having the photo diode 2 as a receiver. The large waveguide 19 dimension (greater than 15 μm in some embodiments) also allows light to couple into and out of optical fibers for out-of-chip communication.

The optical bench 100 on substrate structure can provide better CTE matching and/or heat dissipation for optical components such as the laser diode 1 and the photo diode 2 mounted on the substrate 3 when the substrate 3 comprises semiconductor materials such as silicon, compared to other substrate or interposer materials such as ceramic or organic polymer. More robust and cost efficient integration of optics using silicon micro-fabrication technology is achieved by the optical bench 100 on substrate compared to some other assembly using discrete optical components. Also, there is less crosstalk among optical channels by using the optical waveguide 19 for secure data transfer.

Furthermore, by configuring the optical bench 100 as a transmitting portion (e.g., the portion on the left side of the line 22 and having the laser diode 1 as a transmitter), or as a receiving portion (e.g., the portion on the right side of the line 22 and having the photo diode 2 as a receiver), it is possible to add optical input/output off the package. This optical bench 100 on substrate platform offers higher data rates inside the package by integrating optical components and optical options for signal input and output.

FIGS. 2A-2E are schematic diagrams of various steps of fabrication process of the optical bench on substrate in FIG. 1 according to some embodiments. In FIG. 2A, the RDL 7 is formed over the dielectric layer 6, e.g., by physical vapor deposition (PVD), for metal routing and metal traces for high speed electrical signal propagation. The passivation layer 8 (e.g., silicon nitride or oxide) is deposited afterward for metal protection, e.g., by chemical vapor deposition (CVD). The passivation layer 8, the dielectric layer 6, and the etching hard mask 4, e.g., silicon nitride or silicon oxide, are removed from an area where the trench 21 is to be formed.

In FIG. 2B, the trench 21 (having at least 30 μm depth in some embodiments), including the sloping sides with a slope angle θ, is fabricated by wet etching using KOH(aq)/IPA or TMAH solution. One method to control the anisotropic wet etching is achieved by using KOH (25 wt %-35 wt %) with no less than 5 wt % IPA quantity. The temperature is kept at about 60° C.-70° C. during the wet etching to achieve a reasonable etch rate of 0.2-0.6 micron per minute during the wet etching and to prevent excessive hillock formation.

The reflector layer 5 having sloping sides with a slope angle θ (e.g., 42°-48°) is formed on the trench 21. This step may include depositing an adhesion dielectric layer, then a barrier/adhesion metal layer, such as Ti or Cr, and finally a highly reflective metal such as Al, Cu, Ag, or Au with a thickness greater than 50 nm to achieve reflectivity greater than 90% in some embodiments. The deposition process can be performed by physical vapor deposition (PVD) or electroplating, for example. Any other suitable reflective material or process can also be used.

In FIG. 2C, the waveguide 19, e.g., polymer, for the optical path inside the trench 21 is formed. This step includes forming the bottom cladding layer 9 (e.g., dielectric or polymer) by chemical vapor deposition (CVD) or a coater (for dielectric or polymer), then the core layer 10 (e.g., polymer), and the top-cladding layer 11 (e.g., dielectric or polymer) in some embodiments. The waveguide 19 can be defined by etching and unnecessary portion of the reflector layer 5 can also be removed. In other embodiments, a portion of an optical fiber can be placed in the trench 21 as the waveguide 19.

In FIG. 2D, the UBM layer 12 such as Cu/Ni is formed, e.g., by evaporation or sputtering, or by chemically plating. A solder bump layer 13 is formed or placed on the UBM layer 12 in many ways, including evaporation, electroplating, printing, jetting, stud bumping, and direct placement.

In FIG. 2E, the laser diode 1 and the photo diode 2, as well as other driver or transimpedance amplifier (TIA) chips, are flip-chip mounted (and/or wire-bonded as necessary) over the substrate 3. In FIG. 2E, the portion on the left side of the line 22 is the transmitting portion of the optical bench shown in FIGS. 2A-2D, and the portion on the right side of the line 22 is a receiving portion that can be fabricated in the same or similar process flow described with respect to FIGS. 2A-2D.

FIGS. 3A-3C are schematic diagrams of various steps of another fabrication process of the optical bench on substrate in FIG. 1 according to some embodiments. In FIG. 3A, the etching hard mask 4 and the dielectric layer 6 are formed over the substrate 3. The etching hard mask 4, e.g., silicon nitride or silicon oxide, and the dielectric layer 6 are removed from an area where the trench 21 is to be formed.

In FIG. 3B, the trench 21 with the slope angle θ (as shown in FIG. 1) is formed by etching, e.g., using KOH(aq)/IPA or TMAH solution. One method to control the anisotropic wet etching is achieved by using KOH (25 wt %-35 wt %) with no less than 5 wt % IPA quantity. The temperature is kept at about 60° C.-70° C. during the wet etching to achieve a reasonable etch rate of 0.2-0.6 micron per minute during the wet etching and to prevent excessive hillock formation.

The reflector layer 5 having a slope angle θ (e.g., 42°-48°) is formed on the sloping side of the trench 21. This step may include depositing an adhesion dielectric layer, then a barrier/adhesion metal layer, such as Ti or Cr, and finally a highly reflective metal such as Al, Cu, Ag, or Au with a thickness greater than 50 nm to achieve reflectivity greater than 90% in some embodiments. The deposition process can be performed by physical vapor deposition (PVD) or electroplating, for example. Any other suitable reflective material or process can also be used. The reflector layer 5 can be removed in areas where it is not necessary by a lithography process in some embodiments.

In FIG. 3C, the RDL 7 is formed and defined for metal routing and metal traces for high speed electrical signal propagation. Additional dielectric layer for electrical isolation and microwave confinement can be formed as necessary. After the step in FIG. 3C, the process flow can proceed to the operation of forming the passivation layer 8 (e.g., silicon nitride or oxide) for metal protection as described with respect to FIG. 2A, and then proceed to the steps described in FIGS. 2C-2E afterwards.

According to some embodiments, an optical bench on substrate includes a substrate and a trench formed inside the substrate having a sloping side. A reflector layer is formed over the sloping side. An optical component is mounted over the substrate. The reflector layer is configured to reflect an electromagnetic wave to or from the optical component.

According to some embodiments, a method includes forming a trench inside a substrate wherein the trench has a sloping side. A reflector layer is formed over the sloping side. An optical component is mounted over the substrate, wherein the reflector layer is configured to reflect an electromagnetic wave to be coupled to or from the optical component.

A skilled person in the art will appreciate that there can be many embodiment variations of this disclosure. Although the embodiments and their features have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosed embodiments, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure.

The above method embodiment shows steps, but they are not necessarily required to be performed in the order shown. Steps may be added, replaced, changed order, and/or eliminated as appropriate, in accordance with the spirit and scope of embodiment of the disclosure. Embodiments that combine different claims and/or different embodiments are within the scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure. 

What is claimed is:
 1. An optical bench on substrate, comprising: a substrate; a trench formed inside the substrate and having a sloping side; a reflector layer formed over the sloping side; an optical component mounted over the substrate and outside the trench, wherein the optical component is a laser diode; and a redistribution layer over the substrate, wherein a sidewall of the redistribution layer proximal to the sloping side of the trench is recessed from a first sidewall of the optical component, and another sidewall of the redistribution layer distal from the sloping side of the trench is extended beyond a second sidewall of the optical component opposite the first sidewall; and a passivation layer over the redistribution layer and surrounding sidewalls of an under bump metallization (UBM) layer between the optical component and the redistribution layer, wherein the reflector layer is configured to reflect an electromagnetic wave from the optical component.
 2. The optical bench on substrate of claim 1, wherein the substrate comprises silicon.
 3. The optical bench on substrate of claim 1, wherein the optical component is flip-chip mounted over the substrate.
 4. The optical bench on substrate of claim 1 further comprising, a photo diode configured to receive the electromagnetic wave reflected from the reflector layer.
 5. The optical bench on substrate of claim 1, further comprising a waveguide formed inside the trench wherein the waveguide is configured to guide the electromagnetic wave from the reflector layer.
 6. The optical bench on substrate of claim 5, wherein the waveguide comprises an optical fiber.
 7. The optical bench on substrate of claim 1, further comprising at least one through substrate via formed through the substrate.
 8. The optical bench of claim 7, wherein the through substrate via is electrically connected to the redistribution layer.
 9. The optical bench on substrate of claim 1, further comprising a dielectric layer between the redistribution layer and the substrate.
 10. The optical bench on substrate of claim 1, wherein the sloping side has a slope angle ranging from 42° to 48°.
 11. The optical bench on substrate of claim 1, wherein the reflector layer comprises at least one of Al, Cu, Ag, or Au.
 12. A method, comprising: forming a trench inside a substrate wherein the trench has a sloping side with a slope angle ranging from 42° to 48°; forming a reflector layer over the sloping side; depositing a redistribution layer over the substrate; disposing an under bump metallization (UBM) layer over the redistribution layer; depositing a passivation layer over the redistribution layer and surrounding sidewalls of the UBM layer; and mounting an optical component over an uppermost portion of the substrate, wherein the optical component is electrically connected to a through substrate via (TSV) extending through the substrate, wherein the reflector layer is configured to reflect an electromagnetic wave from the optical component, and wherein the optical component is mounted outside the trench.
 13. The method of claim 12, further comprising forming a waveguide inside the trench wherein the waveguide is configured to guide the electromagnetic wave from the reflector layer.
 14. The method of claim 13, wherein the waveguide comprises polymer.
 15. The method of claim 12, further comprising placing an optical fiber inside the trench as a waveguide to guide the electromagnetic wave from the reflector layer.
 16. The method of claim 12, wherein the substrate comprises silicon.
 17. The method of claim 12, wherein the optical component is flip-chip mounted over the substrate.
 18. The method of claim 12, wherein the reflector layer comprises at least one of Al, Cu, Ag, or Au.
 19. The optical bench of claim 13, wherein the waveguide covers an entirety of a bottom surface of the trench.
 20. An optical bench on substrate, comprising: a silicon substrate; a trench formed inside the substrate and having a sloping side; a waveguide formed inside the trench; a laser diode mounted over the substrate; a photo diode mounted over the substrate; a first redistribution layer over the substrate; a second redistribution layer over a backside of the substrate; a first through substrate via (TSV) extending through the substrate and contacting a portion of the first redistribution layer and a portion of the second redistribution layer at a first side of the trench, wherein the laser diode is electrically connected to the first TSV; a second TSV extending through the substrate and contacting another portion of the first redistribution layer and another portion of the second redistribution layer at a second side of the trench opposite the first side, wherein the photo diode is electrically connected to the second TSV; and a reflector layer formed over the sloping side and configured to reflect an electromagnetic wave transmitted from the laser diode to the waveguide which is configured to further couple the electromagnetic wave to the photo diode. 